The lambda value of a fuel mixture is controlled in order to adjust optimal converting conditions for a catalyzer which is mounted in the exhaust channel of an internal combustion engine. The conversion takes place only in a narrow range of lambda values. Where the center of the range best lies is dependent upon the particular operating condition. This is the case because for different operation conditions, the different toxic substances, that is carbon monoxide, carbon dioxide and nitrogen oxide, occur in different concentrations and because the conventional catalyzers best convert these toxic substances at different lambda values into non-toxic gases. Accordingly, nitrogen oxide is optimally converted at lambda values which are richer than the stoichiometric value; whereas, carbon monoxide and hydrocarbons are better converted in the lean region. Catalyzers are primarily operated in the slightly rich region since the elimination of nitrogen oxide is the primary aim.
The concentration of carbon monoxide is based essentially on heterogeneous mixture distributions and on fluctuations of the mixture composition from cycle to cycle. The mentioned effects influence also the emission of hydrocarbons which, in addition, is greatly dependent upon the combustion temperature and increases with falling combustion temperature. In contrast, the emission of nitrogen oxides decreases with decreasing combustion temperature. The mixture distribution and fluctuations thereof as well as the particular combustion temperature are dependent upon the engine speed and the load. The different composition of toxic substances with different operating conditions requires the adjustment of different lambda values for the different operating conditions.
Different lambda values can be adjusted in that at least one control parameter of the means used for two-level control is changed. This measure is described in DE 25 45 759 Al (U.S. Pat. No. 4,210,106). For practical applications, for example, an extended integration time in the direction rich is stored in a characteristic or in a characteristic field addressable via values of operating variables.
German published patent application DE-OS 32 24 347 discloses an arrangement for reducing the exhaust gas toxic components of an internal combustion engine having an oxygen probe upstream and downstream of the catalyzer. The probe arranged downstream functions as the guide probe to effect a follow-up control of a mean lambda value when there is a deviation from the optimal composition of the air/fuel ratio. This is obtained by modifying the control parameter.
It has been shown that by taking the described action in practice, it is not always possible to precisely adjust that mean lambda value which leads to the optimal conversion rate for the different toxic substances for a particular operating condition which is present.
The invention solves the problem of providing a method for controlling the lambda value with which the desired mean lambda value can be adjusted with great precision for all operating conditions. The invention also has the object of providing an apparatus for carrying out a method of this kinds.